The present invention relates to punches and apparatus for perforating film material such as polyethylene film or other films made of plastic resin. The invention is more particularly directed to an improved punch or slitter-punch and adapter combination. The invention is specifically directed to a hold-down clamp for holding the plastic film steady as the punch acts to form a hole, and which raises to release the film when the punch head is raised.
Blades, punches, and cutters for forming holes and slits in plastic film material are described in prior U.S. Pat. Nos. 4,748,920 and 4,723,466 of Earl T. Pottorff. There, a self-sharpening hole punch formed of a low-friction, semi-rigid material, such as a polymer or a similar plastic resin. The hole punch has a thread at its upper end for screw mounting onto reciprocating apparatus, which may be actuated by a pneumatic cylinder. Slit cutters are separately mounted on the reciprocating mechanism and spaced a small distance from the hole punch to create a gap in the film between the resulting hole and slit.
A so-called quick-slip punch adapter for a plastic bag making machine is described in U.S. Pat. Nos. 5,035,165 and 5,114,394. In that arrangement, a slitter blade is held in a clamp that also fastens to the shaft or rod that holds a hole cutter. However, the slitter blade is a separate element and has to be changed out separately from the hole punch. Also, this requires the operator to employ tools to remove the worn blades and punches, and to replace them with fresh ones. This operation can take several minutes for each hole punch and slitter blade. As there are several of these on each bag machine, considerable down time may be required to replace the worn blades and cutters.
Also, when the space between the slitter blade and the hole punch is to be changed, this must be done by hand and the gap between the slitter and the hole punch must be carefully measured. Moreover, the slitter blade itself has always been made of steel, and suffers the problems of dulling and wear that characterize steel cutting heads, which problem is discussed in the above-mentioned U.S. Pat. Nos. 4,748,920 and 4,723,466. Consequently, the industry has long sought a solution to the problem of how to effect rapid change out of both hole punch and slitter blade, how to effect proper alignment of the parts in the desired spacing, and how to increase the wear life of the parts.
U.S. Pat. No. 6,148,710 discloses a slitter and hole punch and quick change adapter combination which fits a bag machine or similar equipment for punching holes and slits in a web of plastic film material. In that arrangement, a hole punch and a slitter blade are unitarily formed on a base member. Left and right resilient arms extend from distal to proximal, and have free ends that project proximally of the base member. The quick adapter can be an extruded block with a T-channel formed on a lower side with a passage or gap to accommodate the hole punch and the slitter blade. The T-channel has inwardly directed flanges that create side recesses to accommodate the resilient arms of the slitter-and-hole-punch. The side recesses have cooperating hollows to receive detents of the associated resilient arm. The slitter-and-hole-punch can be slid in and out of the T-slot, and when slid in, the detents on the resilient arms keep it in position. The slitter-and-hole-punch can be changed out in a few seconds by hand. The punches can be color coded for barrel diameter, gap size, etc.
A combination of a slitter punch with a hold-down plate arrangement is described in U.S. patent application Ser. No. 09/713,392, now abandoned. This arrangement includes means for clamping the film in place momentarily while the punch mechanism is fired, and then releasing the film immediately thereafter. In the arrangement described in that patent application, there is one air cylinder used for driving the punch, and two additional cylinders needed for driving the clamping plate. The actuation of the various cylinders is synchronized in that case by selecting the clamp plate cylinders to be of smaller size than the cylinder that drives the punch cutting head.
In other designs, a spring is used to bias between the cutting head and the frame on which the hold-down plate is mounted. This arrangement drives the hold down plate against the base plate or backing plate with the downward motion of the cutting head. The result is that the spring is compressed on each downward stroke, and this can lead to spring failure due to fatigue. In addition, the spring bias is directed against the cutting motion of the cutting head, which slows down the motion of the cutting head during a punching operation. As a result the air cylinder has to be oversized by an amount to counter this spring force.